The present invention relates to an image recording device which is applied to a printer, a facsimile machine, a copying machine, and the like, particularly to an ink jet type image recording device which carries out image recording by jetting a hyperfine ink droplet on a recording medium.
Conventionally, an image recording device is generally applied to a printer, a facsimile machine, a copying machine, and the like. As a method of recording image in such an image recording device, several types of the methods, such as an electrophotographic type, an ink jet type, a thermoelectric type, and the like thereof have been put into practical use. Among these types of the methods, the ink jet type has an advantageous feature that enables downsizing the device, low cost fabrication, and color image recording. As a result, it is a recent trend that the ink jet type image recording device is mainly used as a color image recording device in personnel use.
As will later be described more in detail, a conventional color ink jet recording device includes a plurality of recording heads each jetting a respective color ink droplet. As a principle for jetting the ink droplet from each recording head, several systems, such as a system using a piezoelectric actuator, a system using a boiling phenomenon of ink, a system using an electrostatic absorption of ink, or the like, are proposed and practically used.
In the interim, it has currently been a big theme to improve quality of image in the aforesaid ink jet recording device. In other words, a conventional ink jet recording device has already reached the practical limit of recording resolution thereof [for example, approximately 600 through 1200 dpi (dot per inch)]. In addition, a diameter of every ink droplet is generally kept constant in the conventional ink jet recording device. As a result, it is difficult to freely control gradation in each pixel of the conventional ink jet recording device. Accordingly, the conventional ink jet recording device has also reached a certain limit in image quality of output image thereof. A sufficient quality of image cannot be obtained, when a pictorial image is output by the conventional ink jet recording device.
As a method for improving the above-mentioned quality of image, a large number of consideration is made about a method of gradually recording image for varying gradation in each pixel by controlling the diameter of every ink droplet jetted onto the recording medium. Several methods are proposed to modulate a diameter of every ink droplet in the ink jet recording device. An example is disclosed in Unexamined Japanese Patent Publication No. 173654/1991. In the example disclosed therein, a diameter of every ink droplet jetted from a recording head using a boiling phenomenon of ink is modulated by varying an input energy supplied to a heating element provided in the recording head. Further, another example is disclosed in Unexamined Japanese Patent Publication 173654/1991. The another example is an ink droplet jetting device which uses a surface wave interference and is advantageous in modulating a diameter of an ink droplet. Thus, it becomes possible that gradation in each pixel is controlled to record gradual image therein by modulating a diameter of an ink droplet.
However, it is known that reduction of sharpness of image or deterioration of color reproduction is easily caused to occur in the above-described conventional devices. These problems have been confirmed by the inventor of the present invention, et al as the result of actually recording the modulation in the conventional devices. Accordingly, it becomes clear that high quality of image cannot be obtained up to their expectation by the conventional devices.
The inventor has inspected a reason of the problems. As a result, it becomes clear that a ballistic path of an ink droplet jetted from the recording head in the conventional devices varies dependent on a diameter of the ink droplet, so that deviation of points of impact of ink droplets on a recording medium is inevitably caused to occur. This is clearly a main reason why quality of image is deteriorated in the conventional devices. Namely, an ink droplet jetted from the recording head flies under the influence of air flow or air resistance between the recording head and a recording medium. The influence depends largely upon the diameter of the ink droplet. Consequently, the aforesaid ballistic path of the ink droplet largely varies dependent on the diameter of the ink droplet. Thereby, large deviation of points of impact of ink droplets on the recording medium is inevitably caused to occur. In the above-mentioned conventional devices, jetting speed of the ink droplet is not particularly controlled. Therefore, points of impact of ink droplets are deviated with a large error dependent upon the diameter of the ink droplet. Accordingly, it is inevitable that reduction of sharpness of image or deterioration of color reproduction is caused to occur in the above-described conventional devices.